The present invention relates to a process for removing contaminant mercury ions from liquid streams, especially aqueous streams, using manganese oxide-based and metallomanganese oxide-based ion-exchangers.
Industrial processes such as mining and natural gas exploration produce waste waters that contain Hg2+ that are released into the environment. Legislation is restricting these releases of toxic Hg and improved methods are needed for removal of mercury before these waters can be returned to the environment. Ion-exchange is one method by which Hg2+ may be removed from a liquid stream. Many of these Hg2+ contaminated streams will contain significant amounts of metals derived from minerals, including Ca2+ and Mg2+, which are quite common and may compete with Hg2+ for ion-exchange sites on an adsorbent. In these situations, Hg2+ selectivity is important for an adsorbent to function and so that its capacity for Hg2+ removal is not severely diminished by competing cations.
Manganese-based oxides have been utilized as oxidants, ion-exchangers and supports for the remediation of both liquid and gaseous streams containing Hg. In the role of support, sulfur and halogen loaded hydrous manganese oxides are used to remove both metallic Hg and Hg2+ from aqueous solutions (See US 20120103907 A1). This application discusses the use of delta and beta manganese oxides, and pyrolusite, which are derivatized by ammonium and alkali bromides, iodides and sulfides via a redox reaction to accomplish the loading. Hg2+ removal is accomplished by the supported species. Manganese oxides can play the role of oxidant in multicomponent adsorbents, such as is disclosed in JP 2010015858A. This adsorbent consists of 20-40% manganese dioxide, 40-65% Fe2O3, 10-20% silica, alumina, or titania, and 0.5-5% Ag. This adsorbent removes Hg, As and S from hydrocarbon streams via oxidation and adsorption of the oxidized products. Similarly, U.S. Pat. No. 7,655,148 B2 and US 20120024799 A1 disclose two component adsorbents that consist of oxidant and adsorbent components, the oxidants consisting of manganese dioxide or iron oxide and the adsorbents consisting of zirconium oxide, titanium oxide, and iron oxide. The manganese oxide utilized was the potassium manganese oxide synthesized from KMnO4 and MnSO4, which has the hollandite structure, a rectangular one-dimensional tunnel structure with two MnO6 octahedra lining each side of the tunnel. This structure is often designated as a 2×2 tunnel structure. The adsorbent composites were demonstrated on arsenite remediation from aqueous solutions, in which arsenite (As3+) was oxidized to arsenate (As5+), with the anionic arsenate being picked up by the titania, zirconia or iron oxide adsorbent. One problem with this mode of operation was the loss of the manganese oxide adsorbent as soluble Mn2+ was generated by the oxidation process. Attempts to stabilize the potassium manganese oxides from Mn loss were made doping with Fe3+. These materials were also shown to remove Pb2+ from water; the suggested mechanism was ion-exchange with protons on the manganese oxide-iron oxide composite. It was suggested that these materials may also be useful for the removal of cationic and metallic Hg from aqueous solutions. An apparatus and method of metals removal from both gaseous and aqueous streams is disclosed in US 20100059428 A1, which focuses on manganese dioxide based adsorbents. The apparatus is versatile and designed to control a variety of synthesis parameters. No specific manganese oxides are claimed beyond those with a Mn oxidation state between than 3 and 4. Examples included the oxidation of Mn2+ salts in KOH with K2S2O8, which likely yield potassium-containing layered birnessites via the topotactic oxidation of Mn(OH)2. The target pollutants studied with this system were Pb, Fe, Cr and Cu, both in metallic and cation forms, and arsenate and arsenite anions. Two related patents describe more of the workings of this apparatus designed to make manganese oxide-based adsorbents and their use, see U.S. Pat. No. 7,488,464 B2 and U.S. Pat. No. 7,419,637 B2. The removal of radioactive Hg2+ from aqueous solution by γ-Mn2O3, which has a defect spinel structure, has been disclosed (See JOURNAL OF COLLOID AND INTERFACE SCIENCE, 279 (2004), 61-67). The basis for the Hg2+ removal was a pH dependent surface charge and up to 96% of the Hg2+ was removed from solution, depending on conditions. Likewise, so-called manganese dioxide nanowhiskers adsorb Hg2+ via a similar mechanism, as revealed in CHEMICAL ENGINEERING JOURNAL, 160, (2010), 432-439. This nanowhisker material is produced via the reduction of KMnO4 with ethanol. The material is said to have the layered birnessite structure, possess a Mn4+ oxidation state, but there is no K+ incorporated between the layers as in traditional birnessites. The mechanism of Hg2+ uptake is believed to be physisorption, which is pH dependent and operates best in the pH range of 6-9. In other work, OMS-2, which is the K-containing 2×2 tunnel structure (hollandite structure) discussed above, and OL-1 (a birnessite layer) octahedral molecular sieves were looked at for their ability to pick up Pb2+, Cu2+, Ni2+ and Hg2+ from solution via ion-exchange (See Guang Pu Xue Yu Guang Pu Fen Xi (2012), 32 (10), 2842-2846). Both the OL-1 and OMS-2 used in the study are prepared in the K+ form. It was generally found that the tunnel structure of OMS-2 was more effective at removing metals than OL-1, removing 94% of Hg2+ from aqueous solution. Another study looked at the capability of OMS-1 (Mg2+ form, 3×3 todorokite structure, derived from buserite layers) and OMS-2 (K+ form, 2×2 tunnel hollandite structure, derived from potassium permanganate oxidation of a Mn2+-containing solution) to remove Cu2+, Ni2+ and Cd2+ in the presence of Ca2+ and Mg2+ (See JOURNAL OF HAZARDOUS MATERIALS, (2010), 180 (1-3), 234-240). Both materials showed good selectivity for the uptake of Cu2+, the adsorption of which was not bothered by the presence of Ca2+ and Mg2+. OMS-1 was found to be somewhat but considerably less effective for Cd2+ removal and OMS-2 was less effective for Ni2+ uptake and not effective for Cd2+ uptake.
In U.S. Pat. No. 5,637,545, which is incorporated by reference, it is shown that manganese oxides and metal-substituted manganese oxides with the 2×2 tunnel hollandite structure can be prepared via the reduction of permanganates with excess acetic acid under mild conditions. The hollandite structure was formed for these materials in the presence of K+ or NH4+ cations, such as K1.12Mn8O16 in Example 1 of that patent. This reduction with excess acetic acid was useful in that it only reduced the pure manganese oxides to a certain extent, to an oxidation state just below Mn4+, where the ion-exchange capacity was in the range of 1-1.2 cations (K+ or NH4+)/8 Mn or A0.125-0.15Mn, where A+ is the cation that can be exchanged. In the metal substituted manganese oxides forming the hollandite structure, the exchanging ion content was found to vary from 0.7-1.2 A+/8 Mn for an ion-exchange capacity of A0.09-0.15Mn, or A+/Mn=0.09-0.15, when A+ is singly charged.
In contrast to the Hg2+ remediation efforts previously disclosed, the present invention discloses a process for removing Hg2+ from aqueous solution via ion-exchange using unique manganese oxide-based and metallomanganese oxide-based ion-exchangers that are selective for removing Hg2+ ions from liquid streams. The properties of an ion-exchanger that affect its efficacy are a stable ion-exchange capacity, the identity of and the ease with which the resident ion is displaced from the ion-exchanger, and an affinity of the ion-exchanger for the ion to be removed from solution that is greater than that of other competing metal cations that may be present. The present invention has found that ion-exchangers based on manganese oxides and metal substituted manganese oxides that contain the An+ cations H+, Na+, NH4+, Ca2+, Mg2+ and Li+ in which there are 0.08 to 0.25 exchange equivalents per framework metal (Mn+M) will selectively remove Hg2+ ions from solutions that also contain Mg2+ and Ca2+ cations, regardless of the crystal structure, or lack thereof, of the manganese oxide-based or metallomanganese oxide-based ion-exchanger. The ion-exchangers were more effective in these cation forms than K+-containing ion-exchangers prepared under similar conditions.